In this type of camera, the detectors may be either perpendicular for the acquisition of images of certain organs in a 90.degree. tomography, or face to face for the acquisition of the image of the skeletal system on the entire body or 180.degree. tomography.
These cameras may be equipped with one or two TAC (Transmission attenuation correction) devices. The device contains a known radioactive source that emits gamma energy rays; these rays sweep the active surface of the detector through the body of the patient. The attenuation of energy caused by the patient's body is measured in this detector. This measuring results in information that, when treated appropriately, makes it possible to correct the attenuation of the photon energy emitted by the irradiated organ and therefore to obtain a more detailed image.
The radioactive source is a linear rod in a radiation-proof chamber that is however provided with a rectilinear slot through which the radiation is transmitted in the direction of the detector. This slot must be perfectly closed when the TAC device is not in use. Usually, the closing is controlled mechanically and must be totally secured. The TAC device moves parallel to itself in a plane that is parallel to the detector's detection surface.
As it now stands in the technique, the TAC device is only used for certain image acquisitions and in particular for the 90.degree. tomography, meaning when the detectors are perpendicular. In other cases, in particular for the image acquisition using two parallel detectors facing each other, the TAC device is not rotated to operate.